Ambient Temperature and Humidity
Generally, the insulation resistance of materials decreases as ambient temperature and humidity rise. Relatively speaking, surface resistance (or resistivity) is more sensitive to humidity, whereas volume resistance (or resistivity) is more sensitive to temperature. As humidity increases, surface leakage rises, and the conductive current also increases. As temperature rises, the movement rate of charge carriers accelerates, leading to a corresponding increase in the dielectric material's absorption current and conductive current; according to relevant data, the resistance value of a typical dielectric at 70°C is only 10% of its value at 20°C. Therefore, when measuring insulation resistance, the temperature and humidity at which the sample has reached equilibrium with the environment must be specified.
Test Duration
When a specific DC voltage is applied to the material under test, the current does not reach a stable value instantaneously but undergoes a decay process. Upon the application of voltage, a relatively large charging current flows initially, followed by an absorption current that decreases slowly over a prolonged period, eventually settling into a relatively steady conductive current. The higher the resistance value, the longer it takes to reach equilibrium. Consequently, to obtain an accurate reading, the value should be recorded only after the current has stabilized. Standard test methods for communication cable insulation resistance specify that the reading taken after one minute of charging represents the actual measured insulation value. However, this method is somewhat flawed in practice. Since the current flowing through the material is capacitive in nature, it depends on the cable's capacitance; higher capacitance requires a longer charging time-particularly for jelly-filled cables, which require even more time. Thus, for cables of the same type, relying on a fixed one-minute charging time before taking a reading is unscientific given the variations in length and capacitance; this issue warrants further research and discussion.
Intrinsic Cable Factors
Exposure to heat and moisture causes the insulation material of the cable to age, resulting in a decrease in insulation resistance.
